Lead-acid batteries are prone to lead sulfation. Lead sulfation starts when a charging voltage of a fully charged lead-acid battery is removed. Lead sulfate crystals in the battery are converted back to lead during a normal charging cycle. During a normal discharge process, lead and sulfur combine into soft lead sulfate crystals that are formed in the pores and on the surfaces of positive and negative plates inside a lead-acid battery. When a battery is left in a discharged condition, continually undercharged, or an electrolyte level is below the top of the plates or stratified, some of the soft lead sulfate re-crystallizes into hard lead sulfate. This hard lead sulfate is not converted to soft lead sulfate during subsequent recharging. The creation of hard crystals is commonly called permanent or hard sulfation.
When hard sulfation is present, the battery shows a higher voltage than it's true voltage. This may cause a battery to be deemed fully charged by a voltage regulator of a battery charger. This causes the charger to prematurely lower it's output voltage or current, leaving the battery undercharged.
Sulfation accounts for a large percentage of lead-acid battery failures. The longer sulfation occurs, the larger and harder the lead sulfate crystals become. These crystals lessen a battery's capacity and ability to be recharged. Permanent sulfation occurs as the lead-acid battery discharges while in long term storage.
Self-discharge is accelerated by temperature. For batteries that are stored at temperatures over seventy-seven degrees (77°) Fahrenheit/twenty-five degrees (25°) Celsius, the self-discharge rate doubles with an eighteen degree (18°) Fahrenheit/ten degree (10°) Celsius rise in temperature. Thus, sulfation is problem for lead-acid batteries not being used and stored at higher temperatures.
Systems have been designed to charge lead-acid batteries that are deeply sulfated. FIG. 1 is a schematic illustration of a circuit (10) used in prior art battery charging devices for charging lead-acid batteries that are deeply sulfated. The circuit 10 illustrates an alternating current (AC) line input 12 connected to a transformer 14. The transformer 14 outputs a sinusoidal voltage that is applied to a pair of power diodes 16a, 16b. The output of the power diodes 16a, 16b is applied to a pair of silicon-controlled rectifiers (SCR) 18a, 18b. A rectified direct current (DC) output 20 of the SCRs 18a, 18b is applied to terminals (not shown) of a lead-acid battery (not shown). A phase-regulated output controller 22 is connected to the outputs of the power diodes 16a, 16b and the SCRs 18a, 18b. 
One drawback of circuit 10 is that the sinusoidal output voltage of the transformer 14 is applied to the battery terminals at a slowly and steadily increasing rate. This method makes it difficult to break the battery sulfation because a low and steadily increasing voltage does not provide a significant thrust above a voltage of the battery to break the sulfation.
These and other drawbacks exist.